1. Field of the Invention
The present invention is in the field of fiber optic communication systems, and, more particularly, is in the field of amplifiers for use in fiber optic communication systems.
2. Description of the Related Art
In order to satisfy the increasing demand for bandwidth, fiber optic communication systems are moving towards wavelength-division multiplexing (WDM), in which many channels at separate wavelengths are carried on the same fiber. Integral to most current fiber communication systems is the incorporation of erbium-doped fiber amplifiers (EDFAs). When EDFAs are integrated into WDM systems, the gain flatness of the amplifiers becomes critical. For example, a fiber communication line 100 comprising a serial chain of loss sections 110(i) and gain sections 120(i) is shown in FIG. 1A. In FIG. 1A, the loss sections 110(i) are represented as lengths of fiber, and the gain sections 120(i) are represented by EDFAs. Each loss section 110(i) and an associated gain section 120(i) is referred to herein as a xe2x80x9closs-gain section.xe2x80x9d
In a WDM system, multiple independent channels are input at discrete wavelengths. The gain of EDFAs is a function of the wavelength, with a typical unfiltered gain variation appearing in FIG. 1B. After propagation through the first loss-gain section 110(1), 120(1), the channels around 1532 nanometers have higher powers than the other channels due to the EDFA gain variation. Propagating through multiple loss-gain sections causes this disparity in channel powers to grow, and eventually causes the power of some channels out of the last loss-gain section 110(n), 120(n) to drop to unacceptable levels.
To illustrate the foregoing process, the input power spectrum is plotted in FIG. 2A, and the output power spectrum after 5 loss-gain sections (i.e., n=5 in FIG. 1A) is plotted in FIG. 2B. The differences in the two spectra in FIGS. 2A and 2B illustrate the large disparity in resulting channel powers caused by EDFA gain variations.
Many prior solutions to this problem have involved adding a filter to the EDFA to produce a gain-filter section which is flatter than the gain of the EDFA alone. However, fabricating a filter with a correct shape, which is independent of EDFA parameters (e.g., signal and pump powers) and which is stable with time and temperature, is not trivial.
The present invention is directed to gain flattening with nonlinear Sagnac amplifiers. The use of nonlinear Sagnac amplifiers for gain flattening is a novel solution. Instead of using a filter which has a loss which varies as a function of wavelength, the present invention is directed to a filter which has a loss that is a function of power. More particularly, the filter in accordance with the present invention attenuates a specific channel i based on the power of that channel. The filter in accordance with the present invention does not provide a loss that is a conventional broadband power-dependent loss. Rather, the filter in accordance with the present invention provides a narrowband power-dependent loss. In other words, the attenuation of the filter at xcexi, the wavelength of channel i, is a function of the power around that wavelength (i.e., at xcexixc2x1xcex4xcex), but is not a function of the power at a separate wavelength xcexixc2x1n outside of the xcexixc2x1xcex4xcex window. Such a filter is achieved by replacing a standard linear amplifier, as depicted in FIG. 1A, with a nonlinear Sagnac amplifier (NSA), which will be described in detail below.
One aspect of the present invention is an amplification system for reducing power differences in a plurality of output optical signals responsive to a plurality of input optical signals having a plurality of respective optical wavelengths and having a plurality of respective input powers. The amplification system comprises an interferometric loop. A coupler couples the plurality of input optical signals to the loop to cause respective first portions of the input optical signals to propagate in a first direction in the loop and to cause respective second portions of the input optical signals to propagate in a second direction in the loop. The coupler combines the first and second portions after the first and second portions propagate in the loop to produce a plurality of output optical signals. An amplifier is located at an asymmetric location with respect to the center of the loop. The amplifier has a gain spectrum which causes the amplifier to have a plurality of respective gains at the plurality of optical wavelengths. The asymmetric location of the amplifier with respect to the center of the loop causes differences in powers of the first signal portions and the second signal portions of the input optical signals while these portions are traveling through the interferometric loop. The differences in powers of the first and second signal portions cause respective phase shifts in the first and second signal portions to occur in the fiber loop due to the optical Kerr effect. The Kerr-induced phased shifts vary in response to differences in the respective input powers and the respective gains to cause a greater Kerr-induced attenuation of input optical signals having a greater gain-power product. Preferably, the amplifier comprises an erbium-doped fiber amplifier. Certain preferred embodiments further include a wavelength division multiplexed coupler in the loop proximate to the amplifier. A pump source is coupled to the wavelength division multiplexed coupler to provide pump light for the amplifier via the wavelength division multiplexed coupler.
Another aspect of the present invention is an amplification system for reducing output power differences in a plurality of output optical signals responsive to a plurality of input optical signals having a plurality of respective optical wavelengths and having a plurality of respective input powers. The amplification system comprises an interferometric loop which has first and second lengths of optical fiber separated by an optical amplifier. The first length of optical fiber is substantially longer than the second length of optical fiber. A coupler couples the optical signals into the interferometric loop to cause respective first and second portions of the optical signals to counterpropagate in first and second directions in the interferometric loop. The coupler combines the respective first and second portions of the optical signals after propagation through the interferometric loop to produce a plurality of respective output signals at the plurality of optical wavelengths. The plurality of output signals have a plurality of respective output powers. The amplifier has a gain characteristic which causes the amplifier to have a plurality of respective gains at the plurality of optical wavelengths. The first and second portions of the optical signals propagating in the first and second directions experience respective Kerr-induced phase shifts caused by self-phase modulation, by copropagating cross-phase modulation, and by counterpropagating cross-modulation. The location of the amplifier causes light propagating in the first direction to pass through the first length of optical fiber before propagating through the amplifier and the second length of optical fiber. The location of the amplifier also causes light propagating in the second direction to propagate through the second length of optical fiber and the amplifier before propagating through the first length of optical fiber. The location of the amplifier also causes the light propagating in the first direction to experience greater counterpropagating cross-phase modulation than the light propagating in the second direction. The location of the amplifier also causes the light propagating in the second direction to experience greater self-phase modulation and greater copropagating cross-modulation than light propagating in the first direction. The Kerr-induced phase shifts of the plurality of optical signals at the plurality of optical wavelengths are responsive to the respective amplifier gains at the plurality of optical wavelengths and are further responsive to the respective input powers of the plurality of optical signals, such that differences in the output powers caused both by differences in the input powers and by differences in the gains at the plurality of optical wavelengths are reduced.
Another aspect of the present invention is an optical amplification system which comprises a serial chain of at least first and second amplification sections. The first amplification section is coupled to receive a plurality of input optical signals. Each input optical signal has a respective optical wavelength and a respective input optical power. Each amplification section includes a respective amplifier therein. The amplifiers have respective gain characteristics such that gains applied to the optical signals vary with wavelength. The first amplification section provides a first plurality of optical output signals. The second amplification section is coupled to receive the first plurality of optical output signals and to provide a second plurality of optical output signals. Each of the second plurality of optical output signals has a respective optical wavelength and has a respective output optical power. The amplification sections operate to reduce differences in the respective output optical powers of the second plurality of optical output signals caused by differences in the input optical powers and by differences in gains applied to the optical signals. Each amplification section comprises an interferometric loop. The amplifier of the amplification section is located asymmetrically in the loop. A coupler couples light to the interferometric loop to cause the light to propagate as first and second counterpropagating portions at each of the optical wavelengths. The coupler also combines the first and second counterpropagating portions at each of the wavelengths after the first and the counterpropagating portions of the light have propagated through the loop. The first and second counterpropagating portions at each of the wavelengths interfere to provide an output signal at each of the wavelengths. The output signal at each of the wavelengths has a power responsive to input power at the wavelength, responsive to amplifier gain at the wavelength, and responsive to Kerr-induced phase shift at the wavelength. The Kerr-induced phase shift is greater for optical wavelengths having greater gain-power products to at least partially reduce differences in output power caused by differences in gain-power products. Preferably, each of the amplifiers comprises an erbium-doped fiber amplifier. Also preferably, the amplifiers have gains which vary with optical wavelength. The optical signals at the plurality of wavelengths have varying powers. The amplification sections operate to cause output powers at each of the wavelengths at the output of the serial chain to converge toward nominally the same output power within a selectable range of output power.
Another aspect of the present invention is a method of amplifying a plurality of input optical signals. The input optical signals have respective optical wavelengths within a range of optical wavelengths and have respective optical powers within a range of input optical powers. The method produces a corresponding plurality of output optical signals having respective output powers within a selected range of output optical powers. The method comprises passing the input optical signals through a first nonlinear Sagnac amplifier to produce a plurality of intermediate optical signals. The plurality of intermediate optical signals are responsive to the plurality of input optical powers, to amplifier gain and to Kerr-induced phase shift to have a respective plurality of intermediate optical powers within an intermediate range of optical powers. The intermediate range of optical powers is a smaller range than the range of input optical powers. The method further comprises passing the intermediate optical signals through at least a second nonlinear Sagnac amplifier to produce the plurality of output optical signals. The plurality of output optical signals are responsive to the plurality of intermediate optical powers, to amplifier gain and to Kerr-induced phase shift to have a plurality of output optical powers within the selected range of output optical powers. The selected range of output optical powers is smaller than the range of intermediate optical powers and is smaller than the range of input optical powers.
Another aspect of the present invention is an optical system that comprises an input that receives optical input signals having wavelengths within a range of wavelengths. An output outputs optical signals responsive to the optical input signals. At least a first amplifier is interposed between the input and the output. The first amplifier has a first gain spectrum. The first gain spectrum varies with wavelength over the range of wavelengths to cause a first optical signal produced by the first amplifier at a first wavelength within the range of wavelengths to have a first optical power and to cause a second optical signal produced by the first amplifier at a second wavelength within the range of wavelengths to have a second optical power. The first optical power and the second optical power differ by a power difference. The system further comprises at least a second amplifier interposed between the first amplifier and the output. The second amplifier comprises a nonlinear Sagnac amplifier. The second amplifier operates to at least partially compensate for differences in gain of the first amplifier over the range of wavelengths to reduce the power difference between the first optical signal and the second optical signal.
Another aspect of the present invention is an optical system that comprises an input that receives optical input signals having wavelengths within a range of wavelengths and that comprises an output that outputs optical output signals responsive to said optical input signals. A first plurality of amplifiers of a first type are interposed between the input and the output. The amplifiers of the first type have a first gain spectrum that varies over the range of wavelengths to cause a first optical signal at a first wavelength within the range of wavelengths to be amplified with a first gain by the amplifiers of the first type and to cause a second optical signal at a second wavelength within the range of wavelengths to be amplified with a second gain by the amplifiers of the first type. The second gain differs from the first gain. A second plurality of nonlinear Sagnac amplifiers are interposed at selected locations in the optical system. The nonlinear Sagnac amplifiers operate to at least partially compensate for differences in the first gain and the second gain of the amplifiers of the first type to reduce power differences between the first optical signal and the second optical signal.